This invention relates to improvements in reciprocating compressors, and in particular, to an unloading system operable to reduce the load on the compressor motor upon initial startup thereof.
Reciprocating compressors are employed in many varied applications. In a number of such applications, the compressor is designed to operate under a constant load. Accordingly, an electric motor typically employed to drive such compressor may be chosen to provide sufficient torque to handle the load imposed thereon even during initial starting conditions. It is understood, the motor will not provide its maximum torque until it obtains normal operating speed.
In other applications, the compressor operates in systems wherein a variable load is imposed on the compressor. Although the electric motor may be chosen to provide sufficient torque at starting conditions to manage the maximum load, the use of such a motor would not be economical. The relatively few times the motor would be employed under such adverse conditions does not generally warrant the increased cost in providing windings capable of carrying the large flow of current that would occur during the starting period of a compressor having a relatively large load imposed thereon.
The use of a compressor in a refrigeration unit is a typical variable load application. At times when the ambient temperature is at a relatively high level, the condensing pressure of the unit is similarly at a high level. Compressor discharge pressure is slightly above condenser pressure. Accordingly, the load on the compressor is relatively high.
Under heavy load conditions, the motor will draw excessive current in an attempt to produce the necessary torque to obtain operating speed. The excessive current may damage the motor windings, or if provided with an overload protection device, the device may trip to stop the motor. If the load imposed on the compressor is unusually large, the motor may stall producing lock rotor conditions which may result in the total destruction of the motor.
In U.S. Pat. No. 1,607,657, there is disclosed a thermostatic controlled needle valve to permit the cylinder of the compressor to be vented to atmosphere to facilitate starting of the compressor. The use of an expansible bellows in combination with a needle valve provides a rather costly and cumbersome arrangement to obtain the desired reduced load on the compressor during the startup thereof.
It has been determined that, the initial load on the compressor may be effectively reduced by maintaining the discharge valve controlling the flow of compressed fluid from the compressor's cylinder, in an open state to permit the compressor motor to attain normal operating speed. By maintaining the discharge valve in an open state, the pressure differential between the suction side and the discharge side of the compressor is reduced, thereby decreasing the initial load on the compressor.